We have begun to define defects in viral gene expression that restrict replication of "wild-type" laboratory strains of human type A (H1N1) influenza viruses in mouse brain (MB) or mouse embryo brain (MEB) cultures to a single abortive cycle. It appears that the extent of the restriction may be related to the strain of virus as well as to the age of mice or MEB cultures. We wish to continue these studies and extend them to other viral subtypes, with emphasis on comparisons with the recently isolated A/Seal/Mass/1/80 (H7N7) strain. The neurotropic properties of this strain are indicated by its isolation from brains of naturally infected seals and of a variety of experimentally infected mammalian hosts. The neurovirulent potential of the human strain A/WS/33 has been known for a long time, as has that of certain avian strains for naturally or experimentally infected hosts. Genetic analyses of recombinants have defined, with some ambiguity, the nature of specific genes required for expression of neurovirulence. It is our aim to determine in our model systems whether complete viral replication is absolutely required for pathogenic manifestations and whether even abortively replicating viral information can gain access to the CNS via the olfactory neuronal pathway or can block such access of co-infecting neurotropic mutants. We shall: (1) continue analyses of viral gene products expressed in MB and in MEB cultures infected with neurotropic and non-neurotropic influenza viruses, by immunohistological methods and by polyacrylamide gel electrophoresis with immunological identification of specific viral protein bands; relate variable gene expression, specifically of M protein, to age of mice or to stages of cell differentiation in MEB cultures; (2) try to determine possible structural differences between strain-specific M proteins that are expressed in brain cells and those that are not; (3) identify cellular and/or viral functions responsible for non-expression of M protein and relate these to age- or differentiation-specific changes in host systems; (4) identify olfactory neurons as possible targets for productive and abortive infection and as portals of entry to the CNS; (5) determine, by use of biotin-labeled hybridization probes and immunohistological methods, whether viral genomes and their expression can progress trans-synaptically and/or persist in MB or MEB cultures; (6) challenge mice that have survived a primary CNS infection by reinfection with the same or different HN subtypes to see if challenge can evoke immunologically mediated CNS disease.